Phosphate coating for electrical steel



April 24, 1956 H, A. STElNHERZ 2,743,203

PHOSPHATE COATING FOR ELECTRICAL STEEL Filed Aug. 29. 1951 Fig.l.

Heal-Treating l Oven 0 Composition Comprising Phosphoric Acid and Aluminum Hydrate in Water Insulating Film Comprising the Reaction Product of Fig.2. the Steel with a Mixture of Aluminum Hydrate and 40 Phosphoric Acid Ferrous Melal WITNESSES: INVENTOR United States Patent '0 PHOSPHATE COATING FOR ELECTRICAL STEEL This invention relates to an electrical insulating 'coating for silicon steels and the like for use in themaking of laminated cores for electromagnetic apparatus.

f The magnetic. coresv utilized in generators, transformers, electric motors, etc. are preferably made of laminated sheet .steel with a coating of insulating material on each :Hans :A'. Steinherz, Pittsburgh, Pa., assignor'to Westing- 1 house Electric Corporation, East Pittsburgh, :Pa., a corlamination. As is well known in the art, these cores are r 0r laminate form instead of solid steel so that undesirable eddy currents are reduced to a minimum duringuse of the cores.

' In preparing laminations of magnetic material either 'a insulatingmaterial and then cut by a punch'and die. As anincreasednumber 'ofpunchings are made, the cutting edges of both the punch and'die become wornfithat is,

'the edges either become rounded or chipped, depending 'upon'the degree 'of hardness of the'die face. This is particular'ly true when punching laminationsfrom' sheets of silicon steel which is relatively hard. As the punc'hand die edges become worn, small-burrs form on'the cut edges "of-the'punchings. The height ofthe, burr is proportional replaced. The coating vpossesses electrically insulating properties superior to many coatings now applied to punchings for motors and theilike. In 'order to apply on a ferrous magnetic material the thin insulating film or coating of this invention, the sheets are coated with an aqueous mixture containing as its essential'ingredients /2'% to 10% aluminum hydrate, 5% to 50% orthopho'sphoric acid (85% and the remainder being water, the pH of the solution being more acidic 'than018. I

The proportion of aluminum hydrate to phosphoric acid .('H:'PO4) is maintained to provide at least 4.5 parts by weightof the phosphoric acid per part of the aluminum hydrate. The sheets with the applied coating are "then 'pifssed'through an oven where they are heated to a temper'atu're ranging .from 135 C. 11p to 500 C. or higher. The heat drives off the water leaving a thin coating of aluminum phosphate reacted with the ferrous metal of "the'sheet. The time in the oven of course depends upon "the degree of temperature, a'higher temperature requiring "les's'tim'e. "OrdinariIyJa few secondsare sutficient, though longer periods are beneficialin that they assure a complete reaction.

r'oll'of sheet steel or individual sheets are coated with an "tothe'wear'a't the edges of the punch and die. These burrs g are undesirable because they tend to cut through 'the insulatingcoating of the adjoining lamination when stacked,

thereby short circuiting the laminations and nullifying the benefits of the laminatedstructure. Insorneinstances,

While 85% orthophosphoric acid is indicated inithe above formulation, it should be apparent that other strengths of orthophosphoricacid may be used. 'Thus if 'a weaker orthophosphoric acid is used, say 60%, then a proportionate reduction in .the added water is made.

Aluminum hydrate has the general formula AliOan'HzO "Where n is from 2 to 4 orgreater. I have secured good results with aluminumhydrate having the average formula A1203.3H20, also written Al(OH)3.

Sh'e'ets of'various ferrous base metals may be treated in accordance with the invention. Thus silicon-iron sheets "having up to 7% siliconmay be coated therewith. Nickeliron magnetic sheets with up to 85% nickel may be treated. Other magnetic sheets containing a high proportion of iron, alloyed with OIIBIOFIIIOIG other metals may I be treated. The term sheet steel will be employed hereconsiderable damage or destruction of-the core has 'occurred because of .the lack of insulation between lamina tions. Further, theburrmay reach a hei'ghtwliich neces-' sitates an additional grinding step to remove them in order to insure against short circuiting. When the :burr height die must be'either replaced, or its faces reconditioned, :involving great expense and consumption of time.

Anobject-of this invention is thus to provide an improved alumina-phosphate base insulating coating tor sheets of magnetic material, which will enable a :greatly can becomes excessive (about 2 to 4 mils), the :punch and. v

and will in part appear hereinafter. For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing, in which:

Figure 1 is a schematicview, partly in section, of a form of apparatus that may be used in carrying out-the invention, and

Fig. 2 is a fragmentary, greatly enlarged cross-section through a sheet of material prepared in accordance with p the invention.

'in'ztorinclude any of these. I

If necessary,,a wetting agent may be added to the aqueous aluminum phosphate solution. This may be present in the order of from A to 2% of the weight of the "composition, the .amount depending upon the degree of cleanliness of the steel. Examples of' satisfactory wettingagen'ts are the alcohol sulfates, for example,

cHncHz-mcmosoma thelalk y l aryl sulfonates, for example,

' I I vC12I*I25( Cs1 l-i)SOsNa Q and the alkyl sulfonates R CH2 S03 Na, where .R represents an alkyl group such as decyl.

It is not necessary to procure separately the aluminum hydrate and the phosphoric acid and to combine them to form the coating composition. There recently have been made available compositions corresponding to the above formulation of aluminum hydrate and phosphoric acid. These need only be dissolved in water for use.

The following is an example typical of the practice of the invention:

Example A steel strip having 3.25% silicon content and of a 25 mil thickness, was coated with a solution of the following composition, in which all parts are by weight:

Percent Phosphoric acid 31 Aluminum hydrate 6 Water 62 Wetting agent l Patented Apr. 2,4, 1956 The aluminum hydrate was added to the acid-water mixture at a temperature of about 90 C.. It was completely dissolved in about five minutes with the aid of stirring. A clear, light yellow solution was formed.

This solution was applied to the silicon steel Strip in apparatus such as illustrated in Fig. 1 of the drawing. A roll of silicon steel strip 14 mounted on a stand 12 was unwound and the strip 14 was passed over a guide roll 16, thence under a roll 18 mounted within a tank 20 whereby the strip 14 was submerged in solution 22 therein having the above described composition. The strip 14, with its coating of aqueous composition acquired in passing through the solution, was passed through squeeze rolls 24 and 26 composed of a resilient material, such as rubber or felt. These rolls 2426 control the thickness of the layer of the applied solution. The coatingis less than one mil in thickness, though in some cases, more may be applied. The coated sheet strip was then heat-treated in the continuous baking oven 28 in which the air temperature was 485 C., the strip emerging from the oven at a temperature of about 135 C. The speed of the strip through the oven was about 3 ft. per second and the length of the oven was 5 feet. The strip 14 was pulled from the oven 28 by the tension control rolls 30 and 32 and then passed to a punch 34 and die 36.

Referring to Fig. 2 of the drawing, there is illustrated a greatly enlarged cross-sectional view through an insulated steel strip 14 showing an insulating coating as applied by the process. The coating 40 generally has a thickness in the order of 2% or less of the thickness of the sheets.

Because the heat treated insulating coating is so thin, it enables the laminatious to be formed into magnetic cores having a high space factor, that is, containing over 95% magnetic material.

Summarized in the following table are data showing the number of punchings which were made from sheets with different coatings in similar punch and die sets, before the burr height reached 4 mils:

Coating on sheets: Number of Punchings Phosphoric acid-l-aluminum hydrate 285,000 Water glass 100,000 Bare 50,000

From this tabulation, it can be concluded that the phosphate coating increased die life by a factor of nearly three, when compared to water glass, a standard coating material. Compared to bare or uncoated sheet, the coating of this invention enables nearly six times as many punchin gs to be made before the burr height reaches 4 mils.

inspection showed that the punch and die used in the phosphoric acid-aluminum hydrate test to have less Wear than shown by the punch and die used in punching the bare steel up to the same burr height. Therefore reconditioning of the former punch and die would be less expensive.

Dielectric resistance tests of many punchings produced with the insulating coating of the invention have shown a median value of from 20 to 25 ohms per square centimeter per lamination. These resistance values are many times those of other commercially used insulating coatlugs.

The apparatus illustrated in Fig. l is an example of only one means of carrying out the invention. The solution may be applied by spraying, flooding, or flow coating. Thus the tank 20 may be eliminated altogether and the sheet passed between rolls 24 and 26 with the solution being poured or dipped upon the rolls.

Since certain changes in carrying out the above process and certain modifications in the article which embody the invention may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In the process of providing an insulating coating on ferrous sheets, the steps comprising applying to the surface of the ferrous sheets an aqueous solution comprising as its essential ingredients from /2% to 10% by weight of aluminum hydrate, 5% to by weight of phosphoric acid and the balance being water, the

pH of the solution being more acid than 0.8, the ratio of I-IsPOr to aluminum hydrate being at least 4.5 to 1 by weight, and heat-treating the applied solution on the surface of the ferrous sheets at a temperature of from C.

to 500 C. for a period of time sufficient to drive off the water and to react the whole to produce an adherent insulating coating.

2. An article of manufacture comprising a ferrous member and an insulating coating applied to the surface of the member, the coating being composed of the reaction product of the steel with a mixture of from /z% to 10% by weight aluminum hydrate, 5% to 50% by Weight phosphoric acid (85%) the ratio of HsPOr to aluminum hydrate being at least 4.5 to 1 by weight, and the applied mixture being heat-treated at a temperature of at least 135 C. for a period of time sufiicient to cause reaction to take place between the surface of the ferrous member and the applied mixture and not exceeding a temperature at which the coating decomposes.

References Cited in the file of this patent UNITED STATES PATENTS 

